1. Field of the Invention
The present invention relates to an apparatus for producing reduced iron by mixing an iron material and a reducing agent to form a mixed powder, agglomerating the mixed powder to form compacts like pellets, or briquettes and reducing the compacts in a high temperature atmosphere.
2. Description of the Related Art
To produce reduced iron, the first step is to mix an iron ore powder, a coal powder, a fluxstone (limestone) powder, and a binder, and humidify and agglomerate the mixture to form wet compacts called green compacts. Then, the wet compacts are dried to some degree. The dried compacts are heated to a high temperature in a reducing furnace (a rotary hearth furnace, RHF), where iron oxide in the iron ore is reduced with the coal to form reduced iron compacts.
FIG. 13 shows a vertical section of a conventional RHF. FIG. 14 shows a section of a compact supply portion in the conventional RHF.
As shown in FIGS. 13 and 14, a conventional RHF 001 has a pair of parallel rails 003 mounted in an annular formon a circular base 002 installed on a floor surface. To a lower surface of an annular hearth 004, a plurality of pairs of (right and left) wheels 005 are attached along a peripheral direction. The wheels 005 can roll on the rails 003. On the base 002, vertical frames 006 are erected inwardly and outwardly of the rails 003. On an upper portion of the inner and outer vertical frames 006, furnace walls 007 are fixed on both sides of the hearth 004. On top of the furnace walls 007, a ceiling 008 is fixed to cover an area above the hearth 004.
Inside the RHF 001, a space portion S for forming a high temperature atmosphere is defined by the hearth 004, the right and left furnace walls 007, and the ceiling 008. On the furnace walls 007, a multiplicity of burners 009 are provided for heating the space portion S. To the right and left vertical frames 006, water sealing portions 010 by water seal are attached. A lower end portion of a skirt 011 fixed to the furnace wall 007, and a lower end portion of a skirt 012 fixed to the hearth 004 are submerged in the water sealing portion 010.
At a predetermined position of the RHF 001, a compact supply portion 013 for supplying green compacts (raw compacts) onto the hearth 004, and a compact discharge portion 014 for discharging reduced compacts (reduced iron) reduced on the hearth 004 to the outside are provided adjacently. That is, a low ceiling portion 015 is provided detachably in correspondence with the compact supply portion 013. In the ceiling portion 015, a compact acceptance opening 015a is formed. Above the ceiling portion 015, a compact supply hopper 016 and a vibrating feeder 017 having a compact supply port 017a are provided. A low ceiling portion 018 is provided detachably in correspondence with the compact discharge portion 014. In the ceiling portion 018, a compact discharging screw 019 is provided.
Thus, dried green compacts are heaped in the compact supply hopper 016 in the compact supply portion 013, and supplied onto the hearth 004 by the vibrating feeder 017 through the compact acceptance opening 015a via the compact supply port 017a. The hearth 004 rotates at a predetermined speed in the direction of an arrow T in FIG. 14, and forms a high temperature atmosphere upon heating of the space portion S by the burners 009. Hence, while the green compacts on the hearth 004 are moving in the high temperature atmosphere, iron oxide in the iron ore is reduced with the coal to become reduced iron. In the compact discharge portion 014, the reduced green compacts are discharged out of the furnace by the compact discharging screw 019, and packed into a container (not shown).
To produce direct-reduced iron having a high degree of metallization in a reduced iron production process in the RHF 001, it is important to prevent direct-reduced iron after reduction from becoming reoxidized. Thus, a task for discharge from the compact discharge portion 014 is performed such that direct-reduced iron is carried under airtight conditions into the container, and passed on to a subsequent step.
With the conventional RHF 001, the green compacts supplied from the compact supply portion 013 onto the hearth 004 are immediately heated with a high temperature gas inside the furnace. To avoid the situation that the high temperature gas flows reversely and gushes from the compact supply portion 013 (compact supply port 017a, compact acceptance opening 015a), the high temperature gas inside the furnace is discharged through an off-gas duct (not shown) to keep the interior of the furnace at a negative pressure. Hence, when the green compacts are supplied from the compact supply portion 013 onto the hearth 004, outside air F enters the furnace together with the green compacts, and divides into air F1 directed forward in the direction of rotation of the hearth 004, and air F2 directed rearward in the direction of rotation of the hearth 004, i.e., toward the compact discharge portion 014, as shown in FIG. 14. The air F2 directed toward the compact discharge portion 014 contacts the direct-reduced iron to be discharged from the compact discharge portion 014, reoxidizing the direct-reduced iron and lowering the degree of metallization.
As shown in FIG. 13, both sides of the hearth 004 are sealed by the submersion of the skirts 011, 012 in the water sealing portion 010 to prevent the outflow of the high temperature gas inside the furnace, and the inflow of the outside air. As stated earlier, however, air enters the furnace from the compact supply portion 013, because the interior of the furnace is maintained at a negative pressure. This air flows in the entire periphery of the RHF 001 through a space portion Q above the water sealing portion 010, adversely affecting the high temperature atmosphere and the regulation of the pressure inside the furnace.
The present invention has been accomplished to solve the above-mentioned problems. It is an object of this invention to provide an apparatus for producing reduced iron, which can produce reduced iron at a high degree of metallization by preventing entry of the outside air into an RHF, and which can increase the operating efficiency of the RHF.
An apparatus for producing reduced iron according to the invention, designed to attain the above object, is an apparatus for producing reduced iron by agglomerating a mixed powder of an iron material and a reducing agent to form compacts like pellets, or briquettes and reducing the compacts in a high temperature atmosphere, comprising:
a rotary hearth in an annular form and rotatably supported;
a frame for covering an area above the rotary hearth to form a high temperature atmosphere space portion;
a compact supply portion for supplying the compacts onto the rotary hearth;
a compact discharge portion for outwardly discharging reduced iron reduced on the rotary hearth; and
supply portion partitioning means as a partition between the compact supply portion and the high temperature atmosphere space portion.
Thus, even if the outside air enters the furnace from the compact supply portion, the partitioning means suppresses air flow to the high temperature atmosphere space portion and the compact discharge portion, and prevents reoxidation of direct-reduced iron. This makes it possible to produce direct-reduced iron having a high degree of metallization. Also by diminishing the influence on the high temperature atmosphere or the regulation of pressure inside the furnace, the operating efficiency can be increased.
In the apparatus for producing reduced iron according to the invention, discharge portion partitioning means may be provided as a partition between the compact discharge portion and the high temperature atmosphere space portion. Thus, air flow from the compact discharge portion to the high temperature atmosphere space portion is suppressed to diminish the influence on the high temperature atmosphere or the regulation of pressure inside the furnace. Consequently, the operating efficiency can be increased.
In the apparatus for producing reduced iron according to the invention, the supply portion partitioning means may be a partitioning member suspended from a ceiling of the frame and positioned above the rotary hearth. Accordingly, a simple layout can suppress air flow from the compact supply portion to the high temperature atmosphere space portion on the rotary hearth.
In the apparatus for producing reduced iron according to the invention, the supply portion partitioning means may be a partitioning member provided beside the rotary hearth. Thus, air which has entered a gas passage space portion formed beside the rotary hearth can be inhibited from flowing through the entire periphery of the RHF. Consequently, the influence on the high temperature atmosphere or on the regulation of pressure inside the furnace can be diminished.
In the apparatus for producing reduced iron according to the invention, the area above the rotary hearth may be covered by the frame, whereby the high pressure atmosphere space portion may be formed above the rotary hearth, and a gas passage space portion may be formed beside the rotary hearth, a water sealing portion may be provided beside the rotary hearth, a lower end portion of a skirt beside the rotary hearth and a lower end portion of a skirt beside the frame may be submerged in the water sealing portion, and partition plates as the supply portion partitioning means may be provided in the high temperature atmosphere space portion and the gas passage space portion. Thus, air flow from the compact supply portion and the gas passage space portion to the high temperature atmosphere space portion and the compact discharge portion can be suppressed to diminish the influence on the high temperature atmosphere or on the regulation of pressure inside the furnace. Moreover, reoxidation of reduced iron can be prevented.
In the apparatus for producing reduced iron according to the invention, burners for heating the high temperature atmosphere space portion may be provided in the frame, and an off-gas duct may be provided for discharging a high temperature gas to keep an interior of the apparatus at a negative pressure. Thus, the high temperature atmosphere space portion is heated by the burners, and kept at a negative pressure by the action of the off-gas duct. As a result, gas outflow from the compact supply portion and the compact discharge portion can be prevented, so that the high temperature atmosphere space portion can be maintained properly in a predetermined temperature atmosphere.
In the apparatus for producing reduced iron according to the invention, the partitioning member may be formed like a mattress by enveloping bulked ceramic fibers in a woven fabric-like ceramic sheet, and may be attached to the ceiling of the frame. Thus, the partitioning member can be made lightweight, and since the partitioning member is made of flexible materials, its damage due to contact can be prevented.
The apparatus for producing reduced iron according to the invention may include moving means for moving the partitioning member upward and downward, height detecting means for detecting a height of compacts on the rotary hearth, and control means for controlling operation of the moving means in accordance with results of detection by the height detecting means. Thus, even if the height of compacts on the hearth varies with the amount of supply of compacts supplied onto the rotary hearth, the gap between a lower end portion of the partitioning member and the compacts can be constantly maintained at an appropriate level, by adjusting the height position of the partitioning member. Consequently, flow of air inside the furnace can be suppressed reliably, and damage to the partitioning member can be prevented.
In the apparatus for producing reduced iron according to the invention, the partitioning member may be formed by surrounding an entire surface of a steel plate as a core material with an incombustible heat resistant material. Thus, the partitioning member can be prevented from being deformed or damaged.
In the apparatus for producing reduced iron according to the invention, the supply portion partitioning means may be a gas curtain formed by ejecting an inert gas from a gas ejection nozzle formed in the frame toward the rotary hearth. Thus, air flow inside the furnace can be easily suppressed.
In the apparatus for producing reduced iron according to the invention, the partitioning member may be a partition plate of stainless steel having flexibility and capable of elastic deformation. Thus, even if a front end portion of the partition plate contacts a side portion of the rotary hearth, the partition plate elastically deforms, thus preventing damage to the rotary hearth.
In the apparatus for producing reduced iron according to the invention, the partitioning member may be formed like a mattress by enveloping bulked ceramic fibers in a woven fabric-like ceramic sheet, and may be attached to a side wall of the frame. Thus, the partitioning member can be made lightweight, and since the partitioning member is made of flexible materials, its damage due to contact can be prevented.
In the apparatus for producing reduced iron according to the invention, high temperature atmosphere space portion partitioning means may be provided as partitions at least between a heating zone, a CO ratio control zone, and a reducing atmosphere zone in the high temperature atmosphere space portion. Thus, air flow in a side portion of the frame between the respective zones can be suppressed, and the CO ratio in each of the zones can be controlled appropriately. Consequently, reduced iron having a high degree of metallization can be produced.